According to researchers from the Dana-Farber Cancer Institute and Brigham and Women's Cancer Center, Boston, these patients said that this unexpected side effect interfered with their intimate relationships and made them regret the type of treatment they had chosen.

"Prostate cancer is one of the few cancers where patients have a choice of therapies, and because of the range of possible side effects, it can be a tough choice," study leader Dr. Paul Nguyen, a radiation oncologist, said in a Dana-Farber news release. "This study says that when penile shortening does occur, it really does affect patients and their quality of life. It's something we should be discussing up front so that it will help reduce treatment regrets."

The side effect was most common among men who had prostatectomies, which is the surgical removal of the prostate, and those who had hormone-based therapy coupled with radiation.

Nguyen added that most patients are able to cope with just about any side effect if they know about it in advance.

The study involved 948 men with recurrent prostate cancer. The men were enrolled in a registry that collects information on patients whose prostate cancer shows signs of coming back after their first treatment. Most of the men were between the ages of 60 and 80. Of the men involved in the study, 54 percent had their prostate surgically removed, 24 percent received radiation combined with hormone-blocking treatment and 22 percent chose to undergo only radiation.

Overall, 2.6 percent of the men reported their penis was shorter following treatment. However, rates for such complaints changed depending on the type of therapy. For example, about 3.7 percent of men who had had their prostate removed surgically said they thought their penis had shrunk, as did about 2.7 percent of those who had received male hormone-blocking drugs along with radiation therapy.

In contrast, there were no complaints of shorter penis size among men who had received only radiation therapy from either an external X-ray machine or had radioactive seeds implanted directly into the prostate.

The study authors pointed out that the men's penises were not measured before or after treatment — the study's results were based on the men's opinions. The men were also not asked about any changes to their penis size. Instead, they brought this perceived problem to their doctor's attention.

The researchers noted that this side effect of prostate cancer treatment is not well-studied and the issue of shorter penis size might be more common than what this study showed.

"Previous studies have concluded that there is shortened penis length following prostatectomy," study co-author Dr. Jim Hu, a surgeon at the University of California, Los Angeles Medical Center, said in the news release. "This is most common with non-nerve sparing surgery, as this may result in fibrosis and atrophy of erectile tissue due to damage to nerve and vascular structures."

Dr. Luc Cormier, of Dijon University Hospital in France, wrote an editorial accompanying the study. He noted that men's sexual activity should also be examined since it is closely associated with how they perceive the length of their penis.